From the viewpoints of preventing environmental pollution and saving materials and energy, there is definitely a need for low polluting inexpensive paints which can be readily applied to substrates. Particularly preferred as coating applications to substrates which cannot be subjected to high temperatures, for example, construction materials such as plastics, wood, concrete and slate, as well as vehicle bodies to be repaired, are non-toxic, low polluting, high solid contents paints which have a good processability during spray painting even with a reduced solvent content and which are capable of providing a coating exhibiting enhanced properties. As such applications, air-drying paints, such as acrylic paints having incorporated therein cellulosic film-forming materials, for example, nitro-cellulose and cellulose acetate butyrate, as well as cross-linkable paints, such as phthalate and urethane paints, have heretofore been used. However, known air-drying paints pose the following problem in the case where a coating of enhanced properties is required. In such case, the acrylic component in the air-drying paints must have an increased degree of polymerization in order to provide a coating of enhanced properties. Using such a component it is difficult to prepare high solid contents paints having a good processability. Known phthalate paints have a good processability; however, some properties of the coating obtained therefrom, such as hardness, are unsatisfactory. Although known urethane paints have a fairly good processability and can provide a coating of desired properties, such paints are expensive and the isocyanates used therein are toxic.
Recent approaches are based on a cross-linkable water-soluble acrylic copolymer in conjunction with a cross-linking polyepoxide compound. According to Japanese Patent Laid-open Specification No. 50(1975)-134,030, published on Oct. 23, 1975, an acrylic copolymer containing tert.-amino groups and carboxyl groups, derived from 12 to 30% by weight of an ethylenically unsaturated compound having a tert.-amino group, such as dimethylaminoethyl methacrylate, 5 to 15% by weight of an ethylenically unsaturated compound having at least one carboxylic group, such as acrylic, methacrylic, maleic or itaconic acid, and the balance of one or more alkyl acrylates or methacrylates or other copolymerizable comonomers, is water-solubilized by neutralization of the carboxyl groups with an amine, and then combined with a polyepoxide compound. According to Japanese Patent Laid-open Specification No. 51(1976)-30,232, published on Mar. 16, 1976, an acrylic copolymer containing tert.-amino groups, derived from 12 to 30% by weight of an ethylenically unsaturated compound having a tert.-amino group, such as dimethylaminoethyl methacrylate, 0 to 15% by weight of an ethylenically unsaturated compound having at least one carboxylic group, such as acrylic, methacrylic, maleic or itaconic acid, and the balance of one or more other copolymerizable comonomers free from an expoxy group, is water-solubilized by neutralization of the tert.-amino groups with an acid, and then combined with a polyepoxide compound. The coating compositions according to these laid-open specifications are low polluting (because such coating compositions are aqueous) and inexpensive, and can form a coating exhibiting relatively satisfactory properties. However, attainable properties of the coating, such as gasoline and water resistances, gloss and weatherability, are not necessarily well balanced. For example, a coating having a satisfactory gasoline resistance is frequently poor in gloss. Furthermore, it has been found that it is not easy to prepare high solid contents paints of a good processability using the water-soluble acrylic components.
Besides the above-discussed laid-open specifications, Japanese Patent Laid-open Specification No. 52(1977)-32,927, published on Mar. 12, 1977, discloses and claims a composition for coating ship bottoms comprising (A) 70 to 98% by weight of an acrylic copolymer and (B) 30 to 2% by weight of an epoxy compound, which acrylic copolymer is derived from 50 to 98% by weight based on the copolymer of C.sub.6 -C.sub.20 -alkyl or cycloalkyl acrylate or methacrylate, 2 to 20% by weight based on the copolymer of an ethylenically unsaturated compound having a basic nitrogen atom, such as dimethylaminoethyl methacrylate, and, as the balance, one or more other copolymerizable comonomers. The specification teaches that in view of the intended use of the coating in a submerged condition in the sea, the C.sub.6 or higher alkyl or cycloalkyl acrylate or methacrylate component should constitute a major proportion of the acrylic copolymer so as to render the coating sufficiently water repellent.